SREBRENICA, Bosnia (Reuters) - Thousands of Muslim Bosniaks paid respect to 127 victims of the 1995 Srebrenica massacre buried in individual graves on Monday in a ceremony avoided by Serbs after survivors said they were not welcome because they denied genocide had occurred.

Muslim men pray in front of coffins during mass funeral in Potocari near Srebrenica, Bosnia and Herzegovina July 11, 2016. REUTERS/Dado Ruvic

Nationalist Bosnian Serb forces led by General Ratko Mladic executed 8,000 Bosniak men and boys after overrunning Srebrenica near the end of Bosnia’s war 21 years ago and dumped their bodies in pits - Europe’s worst atrocity since World War Two.

Serb forces subsequently dug up the bodies and scattered them in a systematic effort to conceal the crime. U.N. war crimes investigators later excavated the mass graves, but over 1,000 bodies are still missing.

Most Serbs, both in Bosnia and Serbia whose 1990s leadership armed and funded Bosnian Serb forces, strongly deny that the massacre was genocide as judged by the U.N. war crimes tribunal for former Yugoslavia.

They dispute the death toll and the official account of what happened, reflecting conflicting narratives about how and why Yugoslavia broke up in bloodshed. That divide continues to hinder reconciliation and stifle Bosnia’s progress toward integration with Western Europe. The Balkan country today is split into autonomous Serb and Bosniak-Croat entities.

For Muslim Bosniaks, Srebrenica has become a symbol of collective suffering and July burials of victims an annual ritual. July 11, the start of the five-day massacre, was made a national day of mourning by Bosnia’s weak post-war central government comprised of Bosniaks, Serbs and Croats.

Some Serb officials attended previous burial ceremonies but this year was the first time none came after families said that those who deny genocide happened in Srebrenica were not welcome.

“How can anyone say this was not a genocide?” said Nura Suljic, 57, pointing at endless rows of white marble tombstones in the flower-shaped Potocari memorial cemetery near Srebrenica, where more than 6,300 victims are now interred.

Suljic buried her brother after his bones were found in three different mass graves.

Bakir Izetbegovic, the Bosniak chairman of Bosnia’s three-person inter-ethnic presidency and son of its late wartime president, urged Serbs to face up to historical facts.

“Acceptance and recognition of the truth is the first step toward genuine trust,” he said.

Last year’s 20th anniversary was marred when an angry crowd at the ceremony chased away Serbian Prime Minister Aleksandar Vucic, who had enlisted ally Russia to veto a U.N. resolution that would have condemned the denial of Srebrenica as genocide.

Survivors did not want a possible repeat of any such incidents that would distract attention away from the victims.

Serbian and Bosnian Serb officials bristled at the condition for attendance. “That was not genocide and Serbs will never accept that word,” Bosnian Serb President Milorad Dodik said.

Thousands of grieving families stood by green-draped coffins in sweltering mid-summer heat, some kneeling, crying and hugging the caskets before they were lowered into freshly-dug graves.

“All I have been left with are these three cold stones I can hug instead of my two sons and husband, and a grief I will carry in my heart until I die,” said 67-year-old Nezira Memic.